Starch is a polymeric carbohydrate material of very high molecular weight. Its monomeric units, termed anhydroglucose units, are derived from dextrose, and the complete hydrolysis of starch yields dextrose. In the United States, dextrose is manufactured from corn starch; in Europe from corn starch and potato starch; and in Japan from corn starch and white sweet potato starch.
Until 1960, dextrose was prepared from starch by acid hydrolysis. The method of preparation involved heating starch with hydrochloric or sulfuric acid at temperatures of 120.degree.-145.degree. C, then neutralizing the hydrolysis mixture with sodium carbonate, clarifying, and crystallizing the dextrose. Unfortunately, the yield of dextrose is lowered by the formation of relatively large amounts of reversion products, i.e., products which are formed by the recombination of dextrose molecules. Also, because of the high temperature and low pH of the hydrolysis reaction, some of the starch is converted to hydroxymethyl furfural, levulinic acid and color bodies. The formation of such degradation products is irreversible and, to the extent they are formed, the yield of desired dextrose is of course adversely affected. Still further, the use of hydrochloric acid or in some instances, sulfuric acid, and the subsequent neutralization of this acid with alkali results in the formation of inorganic salts which interfere with crystallization of the final dextrose product.
Later, hydrolysis of starch to dextrose was accomplished by means of enzymes. The principal enzyme used for such purpose was, and continues to be, glucoamylase. This enzyme effectively hydrolyzes the starch by cleaving one molecule of dextrose at a time from the starch molecule. As a practical matter, however, it is necessary first to thin the starch before subjecting it to the action of glucoamylase. This thinning step may be accomplished either by means of acid or enzyme. The starch is thinned to a D.E. of about 10-20, then treated with glucoamylase. This two-stage process is referred to as an acid-enzyme process or an enzyme-enzyme process, depending upon the nature of the thinning step employed.
In the case of the acid-enzyme process, the initial acid-thinning step also requires a rather high temperature, i.e., in the order of 120.degree. C. This of course produces starch fragments that readily retrograde, and also produces reversion products. As expected, these occur at the expense of the desired formation of dextrose.
The same is characteristic of the enzyme-enzyme process which also requires a relatively high temperature for the thinning step, viz., 85.degree.-95.degree. C. Furthermore, it is usual practice to heat the thinned starch at still higher temperatures, viz. of the order of 120.degree.-160.degree. C, to complete gelatinization of the starch and to improve filtration. In addition, certain fat-amylose complexes are formed which are quite insoluble and cause filtration difficulties.
None of these processes is entirely free of processing difficulties because of the inevitable presence of retrogradation products, starch-fat complexes and reversion products. To the extent that these are formed, processing difficulties are encountered particularly in the filtration of the product mixture, and the yield of dextrose is diminished.
Wallerstein et al (U.S. Pat. No. 2,583,451) disclose an enzymatic hydrolysis process which does not utilize a high temperature, gelatinization step, but the yields of dextrose are quite low. Leach et al, Cereal Chemistry, Vol. 38, No. 1 January, 1961, pp. 34-46, likewise show the enzymatic hydrolysis of granular starch, with various alpha-amylases, but at low temperatures.